The structuring of surfaces according to predetermined patterns is an elementary step in any device-manufacturing process. Precision, speed and cost of the structuring processes frequently are decisive factors for success or failure of a product. The development of microelectronics, microbiology, and microtechnology in general raised these needs enormously, generating ever increasing requirements for smaller structures, larger scale integration, and lower cost.
The classical patterning techniques used in microtechnology are photo- and electron beam lithography. Photolithography is a fast, efficient parallel process. Its principal problem is the diffraction limit which restricts the minimum structural dimensions to about one half to one quarter of the light wavelength. To cope with the shrinking dimensions of microtechnical structures, imaging systems for shorter and shorter wavelengths were developed in recent years. Due to a number of basic limitations, the ultimate limits of conventional optical lithography will be of the order of 100 nm. These dimensions will be reached soon. Near-field optical lithography is not bound to the diffraction limit and therefore suitable for the generation of even smaller structures. This method, however, still is in a very early state; its potential for industrial application cannot be estimated yet.
Electron beam lithography is the present day's preferred solution for the generation of structures with very small dimensions. As a serial, direct writing process, however, it becomes slower and slower with increasing complexity of the patterns to be transferred. For this reason, electron beam lithography has been used mainly in mask fabrication so far and not in the mass production of semiconductor chips. Ion beam lithography operates on similar principles as electron beam lithography but is far less established because of ion implantation and other disadvantageous effects.
A basic feature of optical and electron lithography is the use of an overlay, typically an organic polymer, that serves as the base for pattern formation on an underlying substrate. The overlay is formed on the substrate by homogenous deposition. Evaporation or spin-coating from an organic solvent provides a continuous film on the substrate. Exposure of the overlay to radiation (optical, electron or ion) causes localized changes in its chemistry permitting differential dissolution of the overlay and opening up windows in the film onto the underlying substrate. Patterning can then be affected by wet chemical or dry etching processes where the presence of the overlay provides a local, externally controlled physical mask to chemical reaction. Alternatively, material can be deposited onto the substrate through the windows in the overlay by various methods such as evaporation, chemical vapor or sputter deposition, or galvanic techniques. These methods of pattern formation are tremendously useful; nevertheless they have certain shortcomings: Several steps are required before pattern transfer is complete; dissolution of the overlay requires a development step that exposes the whole system to organic solvents, plasmas or otherwise chemically harsh conditions. Here, bulk quantities of chemicals are consumed even though only quite localized chemical reactions are needed, steps that are generally wasteful of the reagents. The use of physical masks means that when material is deposited through the mask much of it will be unproductively directed onto the tops of the physical mask. Afterwards, where elimination of the externally controlled physical mask is needed, the conditions for this removal can be injurious to the newly formed substrate, especially where fragile organic materials have been deposited. Furthermore, the masking layer provided by the overlay is not reusable so that specialized equipment is required to form a new pattern on an existing or subsequent substrate. Finally, irradiation used to form the pattern can damage the underlying substrate by the introduction of chemical or electronic disturbances in the region near the overlay.
In view of the increasing gap between the needs of industry and the existence of foreseeable limitations of the established techniques, the development of alternatives is highly desirable. Stamping techniques, including embossing and gravure (intaglio printing) are promising candidates in this context. Ignored for many years in micro-technology, they recently began to attract renewed attention: It was demonstrated that structures with very small dimensions, in some cases of less than 100 nm in size, can be replicated by means of stamping techniques as described, with regard to the use of self-assembled monolayers e.g. in the article by A. Kumar, H. Biebuyck and G. M. Whitesides "Patterning SAMs: Applications in Materials Science", Langmuir 10, 1498 (1994) or by H. Biebuyck, N. B. Larsen, E. Delamarche and B. Michel in "Lithography Beyond Light", IBM Journal of Research and Development 41, 159-170 (1997), with regard to embossing e.g. by Y. Chou, P. R. Krauss and P. J. Renstrom "Imprint of Sub-25 nm vias and Trenches in Polymers", Appl. Phys. Lett. 67, 3114-3116 (1995), and with regard to intaglio techniques e.g. by E. Kim, Y. Xia and G. M. Whitesides "Polymer Microstructures Formed by Molding in Capillaries", Nature, 376, 581-583 (1995) and by E. Delamarche, A. Bernard, H. Schmid, B. Michel and H. Biebuyck "Patterned Delivery of Immunoglobins to Surfaces Using Microfluidic Networks", Science 779-781 (1997). Intaglio techniques, commonly known from gravure, exploit capillary attachment of inks to the cavities of a patterning at the surface of a patterning device. When pressed against the inked patterning device, the paper penetrates the cavities slightly and draws out the ink.
In microtechnology, the substrate to be structured neither is porous nor flexible in general. Furthermore, the desired modification does not necessarily involve the deposition of an ink. It is possible, however, to conceive pattern transfer devices and methods on the basis of intaglio printing techniques which are not restricted to those modes of operation and open interesting alternatives to the conventional lithography while avoiding some of the disadvantages mentioned before.
In the publication by S.P.A. Fodor et al. "Light-directed, spatially addressable parallel chemical synthesis", a method is described which combines solid-phase chemistry, photolabile protecting groups and photolithography to yield a highly diverse set of chemical products. Binary masking yields 2.sup.n compounds in n chemical steps. For each step, the test substrate has to be covered completely with material containing one of the n starting materials. The resulting consumption of starting materials can become considerable if the cost of these materials is high. The method was used successfully in accessing genetic information with high-density DNA arrays as described in the article with the same title by M. Chee et al. which appeared in Science, Vol. 274, 610-614, Oct. 25, 1996.
H. Biebuyck, E. Delamarche and B. Michel disclosed a method in International patent application PCT/IB96/00908 which allows controlled deposition of CDBs by means of a stamping technique and mediated by layers of complementary CDBs (C-CDBs). The CDBs and C-CDBs can be molecules, macromolecules and/or other nanostructures. The proposed method is applicable on (C-)CDBs whose chemical composition is at least partially known at the external region of the body which allows to deposit them with a chemically determinable orientation, as is the case with many molecules or macromolecules or materials or components derivatized on their surface to have a useful chemical asymmetry. Specifically, such (C-)CDBs may be organic molecules acting as ligands or receptors for the respective complementary molecules.
Specifically, Biebuyck et al. suggest to use a stamping means for simultaneous locally-separated deposition of different CDBs whose ridges are "inked" with different types of C-CDBs. The stamping means then is dipped into a fluid which contains the CDBs to which the C-CDBs are complementary. After selective attachment of the CDBs, the stamping means is removed from the fluid and brought in contact with a substrate. The surface of the substrate is covered with an attachment means which exerts strong adhesion on CDBs. The CDBs get attached and hence transferred to the substrate surface in areas predetermined by the pattern of the stamping means. By this procedure, the orientation of the CDBs is guaranteed and the arising functionality is maximised.
In the patent application by Biebuyck et al., the step of inking the stamping means with different C-CDBs, which is a non-trivial one, was left unattended. The present invention provides a simple solution to this problem and at the same time allows to generate in a single step patterns formed by different C-CDBs.
In the above-mentioned publication by E. Delamarche, A. Bernard, H. Schmid, B. Michel and H. Biebuyck "Patterned Delivery of Immunoglobins to Surfaces Using Microfluidic Networks", Science 779-781 (1997), the authors describe the formation of a network of conduits at the interface of a substrate in contact with a printing plate made from an elastomer. The active transfer region of the printing plate is structured into a patterning in such a way that capillaries are formed which can be filled through openings arranged outside the transfer region. The capillaries can direct spatially chemical reactions between the surface of the substrate and ligands introduced by flow of aqueous, buffered solutions through the network, immobilizing ligands, --like drugs, enzymes and immunoglobins--all along the conduits by their covalent attachment to the activated substrate. Release of the elastomer reveals a uniform and functional layer of the ligands in the image of the pattern molded in the elastomer. Subsequent exposure of this substrate to a homogeneous or heterogeneous solution of receptors allows specific recognition and attachment of the receptors to the immobilized ligands with high resolution (sub-micron) and specificity.